Capacitance monitors are well known for determining the moisture content of soil and have a variety of applications. Capacitance monitors have been used to optimize irrigation schedules to conserve precious water resources and to study the movement of water through soils. For example, capacitance monitors are useful for determining moisture penetration under a road bed or around a hazardous waste site.
An example of a moisture sensing probe for use in soils is described in U.S. Pat. No. 4,929,885. The patented probe provides direct contact with the soil and has a tapered probe body that improves contact between the surrounding soil and the electrodes of the probe.
Another type of capacitance monitor, in which the capacitance sensing probe is designed for insertion into an access tube that is embedded in the soil, is disclosed in Bell, J. P., T. J. Dean and M. G. Hodnett, Soil Moisture Measurement By An Improved Capacitance Technique, parts I, II, Journal of Hydrology, 93 (1987): 67-78, 79-90. In this system, a number of access tubes can be installed in a field and the capacitance sensing probe can be inserted successively into different access tubes to determine a moisture profile for an entire field. The accuracy of the profile depends on precise installation of the access tube and on precisely reproducing the position of the probe body within the access tube.
The disclosed probe body is fitted with a polyvinyl chloride ("PVC") extension tube of the same diameter as the probe. A mounting block fitted to the top of the access tube holds a spring loaded key that is said to provide reproducible depth positions by engaging a series of indents spaced along the extension tube. Fabric centralizing rings spaced along the access tube engage the probe as it is moved axially within the access tube to center the probe within the access tube.
However, precise and reproducible radial positioning of a probe within an access tube is critical to obtain accurate results in measuring soil moisture content. The fabric centralizing ring does not always center the probe body with precision and does not take account of variations in the diameter of the access tubes that result from the tolerances with which such tubes are formed or variations from the tolerances themselves.
With the foregoing in mind, it is an object of the present invention to overcome the limitations of the prior capacitance monitors discussed above and to provide a capacitance monitor for determining soil moisture content in which a capacitance sensing probe is inserted into an access tube embedded in the soil with precise and reproducible radial and axial positioning to provide more accurate measurement of soil moisture.